1. Field of Invention
This invention relates tools for the determination of formation properties; particularly, this invention relates to nuclear tools having neutron generators and neutron monitors.
2. Background Art
In hydrocarbon exploration and production, it is important to determine whether an earth formation contains hydrocarbon and how much hydrocarbon is in the formation. Underground hydrocarbons, as well as water, are typically contained in pore space in the formations. Neutron “porosity” tools are traditionally used to determine the amount of hydrocarbon and water present in pore spaces of earth formations because of their unique abilities to detect such fluids.
A neutron tool contains a neutron-emitting source (either a chemical source or a neutron generator) and one or more axially spaced detectors that respond to the flux of impinging neutrons or gamma-rays resulting from the interactions of neutrons with nuclei within the borehole and formation in the vicinity of the borehole. The basic concept of a neutron porosity tool is predicated on the fact that (a) hydrogen is the most effective moderator of neutrons and that (b) most hydrogen found in earth formations is contained in liquid in the pore space of the formation, either as water or as liquid hydrocarbon or gas. For neutrons emitted with a fixed energy by the source, the count rates recorded by the detectors typically decrease as the volumetric concentration of hydrogen (e.g., porosity) increases.
FIG. 1 shows a simplified schematic illustrating a wireline neutron logging operation. As shown in FIG. 1, a neutron tool 11 is disposed in a wellbore 12. The neutron tool 11 includes a neutron source 13 and one or more neutron detectors 14. The neutron source, which may be a chemical source or an electronic neutron generator, emits neutrons into the formation 15 surrounding the wellbore 12. The emitted neutrons traverse the formation 15 and interact with matter in the formation. As a result of such interactions, the neutrons lose some of their energy. Consequently, the neutrons may arrive at the detector 14 with lower energies. By analyzing the response of the detectors to these neutrons, it is possible to deduce the properties of the surrounding formations.
Traditional neutron tools with chemical sources are able to measure the porosity of a formation in the form of a thermal neutron porosity reading. The chemical source typically relies on α-beryllium reactions in a 241Am-Be mixture. The interaction of the alpha particle with the Beryllium results in the release of a neutron. The average energy of the emitted neutrons is about 4 MeV. These high-energy neutrons interact with nuclei in the formation and become slowed mainly by elastic scattering to near thermal energies. The slowing-down process is dominated by hydrogen. At thermal energies, the neutrons diffuse through the material until they undergo thermal capture. Capture is dominated by hydrogen and thermal neutron absorbers, such as chlorine or iron.
FIG. 2A shows one example of a chemical source neutron tool (e.g., CNL® from Schlumberger Technology Corp., Houston. Tex.). As shown in FIG. 2A, the chemical source neutron tool 120 includes a chemical source 125, which includes a radioactive material, such as AmBe. The chemical source neutron tool 120 also includes a near detector 124 and a far detector 122 to provide a countrate ratio, which is used to calculate the porosity of a formation. The near detector 124 and far detector 122 are thermal detectors. In addition, the tool 120 includes shielding materials 123 that prevent the neutrons generated by the chemical sources from directly reaching the detectors, minimizing the interference from the neutron source 125.
Neutron tools using chemical sources have been around for a long time. As a result, users are more familiar with the thermal neutron porosity measurement acquired with chemical source neutron tools. In addition, petrophysicists typically use thermal neutron porosity for specific minerals as part of their formation analysis. However, chemical sources are less desirable due to their constant emission of radiation and strict government regulations. In addition, the material for many of these chemical sources is becoming scarce. Therefore, there is a need to develop neutron tools that do not rely on chemical sources.
In response to the desire to move away from chemical source neutron tools, some modern neutron tools have been equipped with electronic neutron sources, or neutron generators (minitrons). Neutron generators contain compact linear accelerators and produce neutrons by fusing hydrogen isotopes together. The fusion occurs in these devices by accelerating either deuterium (2H=D) or tritium (3H=T), or a mixture of these two isotopes, into a metal hydride target, which also contains either deuterium (2H) or tritium (3H), or a mixture of these two isotopes. In about 50% of the cases, fusion of deuterium nuclei (d+D) results in the formation of a 3He ion and a neutron with a kinetic energy of approximately 2.4 MeV. Fusion of a deuterium and a tritium atom (d+T) results in the formation of a 4He ion and a neutron with a kinetic energy of approximately 14.1 MeV.
These neutrons, when emitted into formations, interact with matter in the formations and gradually lose energy. This process is referred to as slowing down. The slowing-down process is generally dominated by the elastic scattering of neutrons by hydrogen nuclei, and is characterized by a slowing-down length. Eventually, the high-energy neutrons are slowed down enough to become epithermal neutrons or thermal neutrons. Thermal neutrons typically have an average kinetic energy of 0.025 eV at room temperature, while epithermal neutrons typically have energies corresponding to kinetic energies in the range of 0.4-10 eV. However, neutrons with energies as high as 1 keV may be considered epithermal. One of ordinary skill in the art would appreciate that these energy ranges are general guidelines, rather than clear-cut demarcations. The slowed-down neutrons are typically detected by detectors in the tools, which may include fast neutron detectors, epithermal neutron detectors, and thermal neutron detectors.
FIG. 2B shows one example of an electronic source neutron tool (e.g., APS® from Schlumberger Technology Corp., Houston, Tex.). Examples of such tools can be found in U.S. Pat. No. 6,032,102 issued to Wijeyesekera et al., and in U.S. Pat. No. Re. 36,012 issued to Loomis et al. These patents are assigned to the present assignee and are incorporated by reference in their entirety. As shown in FIG. 2B, the electronic source neutron tool 121 uses an electronic neutron source to produce high-energy (e.g., 2.4 or 14 MeV) neutrons. The high-energy neutrons emitted into formations are slowed to epithermal and thermal energies by interactions with matter (nuclei) in the formations. The epithermal or thermal neutrons are detected by detectors on the neutron tool 121, such as near detector 126, array detector 127, and far detector 129. By measuring epithermal neutrons, the detector responses are primarily dominated by the hydrogen content in the formation, without complication from neutron absorbers. Thus, the electronic neutron tool 121 may conveniently provide measurements for hydrogen index. In addition, the neutron tool 121 may also include an array thermal detector 128 to detect thermal neutrons that returned from the formation. The epithermal neutron and thermal neutron measurements obtained with this tool can be used to derive various formation parameters.
Between the chemical source and the electronic source, the chemical source has the advantage of having a stable and predictable neutron output. The change of their neutron output is dominated by the half-life of the primary alpha source used to generate the nuclear reaction. Given the half-life of the alpha sources typically used (e.g., 241Am: T1/2=430 yrs), it is sufficient to determine or verify the neutron output at intervals of several months.
In contrast, the neutron output of an electronic source varies over time due to internal effects in the electronic source and its power supplies. In addition, the neutron output of an electronic source is also influenced by external factors, such as temperature, shock, and vibration. If an electronic neutron source is to be used for absolute measurements, it is necessary to have a device that monitors its instantaneous output.
The need for neutron monitors has been recognized in the past. At present, downhole neutron monitors rely exclusively on scintillation detectors, in particular plastic detectors, for neutron output monitoring. These monitors rely on the proton recoil following elastic neutron scattering in the organic scintillator. Such technologies are described in U.S. Pat. Nos. 6,166,365 and 6,884,994 issued to, both of which are issued to Simonetti et al. and U.S. Pat. Nos. 6,495,837 and 6,639,210, both of which are issued to Odom et al. See also, U.S. Pat. No. 6,754,586, issued to Adolph et al., which discloses monitors for use to calibrate the outputs of electronic neutron sources.
While the prior art scintillation type monitors provide accurate monitoring of neutron outputs form electronic neutron generators, there remains a need for better monitors.